The present invention relates to a direction of arrival estimation method using an array antenna and a radio reception apparatus.
FIG. 1 is a block diagram showing a conventional radio reception apparatus using an array antenna. In FIG. 1, the conventional radio reception apparatus has units 10-1 to 10-N corresponding in number to the channels received.
Reception units 10-1 to 10-N each mainly comprise the same number of despreading sections 12-1 to 12-M as the antenna elements of array antenna 11, direction of arrival estimation section 13 for estimating the directions of arrival of signals based on the outputs of despreading sections 12-1 to 12-M, beam former 14 for forming directivities in the directions of arrival estimated by direction of arrival estimation section 13 and letting the outputs of despreading sections 12-1 to 12-M pass according to the formed directivities and channel reception section 15 for extracting a channel signal from the output of beam former 14. The array antenna has a configuration of a plurality of antenna elements linearly spaced at intervals of half-wavelength of a carrier frequency.
Here, a beam search technique will be explained as an example of the direction of arrival estimation method. An input signal vector X(t) of the antenna is given by following expression (1).
X(t)=[x0(t),x1(t), . . . ,xn(t)]Txe2x80x83xe2x80x83(1)
Furthermore, vector a(xc3xa8) having a peak in a xc3xa8 direction (called a xe2x80x9csteering vectorxe2x80x9d) is given by expression (2).
a(xc3xa8)=[1,exp(xe2x88x92jΠ sinxc3xa8), . . . ,exp(xe2x88x92jnΠ sinxc3xa8)]Txe2x80x83xe2x80x83(2)
By carrying out reception using the above expression, it is possible to observe power in the xc3xa8 direction included in X(t). That is, spatial profile H(xc3xa8) can be determined by following expression (3).
H(xc3xa8)=a(xc3xa8)Txc2x7X(t)xe2x80x83xe2x80x83(3)
From this, a spatial profile of X(t) over a 180xc2x0 sector extending ahead can be determined. Here, xc3xa8 takes a value between xe2x88x9290xc2x0 and 90xc2x0.
Beam forming in an array antenna is obtained by multiplying a reception signal from each antenna element by a complex amplitude and giving an arbitrary directivity using the result thereof.
An example of beam forming of the array antenna will be explained below. FIG. 2 illustrates an example of beam forming of an array antenna. In FIG. 2, base station apparatus 21 has four antenna elements, forms directivities and performs communications with mobile stations 22 and 23.
When base station apparatus 21 communicates with mobile station 22 located in the xc3xa8 direction, the input signal vector of the four antenna elements is given by expression (4) using expression (1).
X(t)=[x0(t),x1(t),x2(t),x3(t)]Txe2x80x83xe2x80x83(4)
Then, when the directivity of the array antenna is directed to mobile station 22 at angle xc3xa8 from base station apparatus 21, steering vector a(xc3xa8) is given by expression (5).
a(xc3xa8)=[1,exp(xe2x88x92jΠ sin xc3xa8),exp(xe2x88x92j2Π sin xc3xa8)]Txe2x80x83xe2x80x83(5)
By carrying out a vector multiplication of expression (4) and expression (5) on the reception signal, the array antenna of base station apparatus 21 forms a directivity with a large gain in the xc3xa8 direction. For example, an array antenna with four linearly spaced antenna elements has a maximum gain in the xc3xa8 direction and the gain at an angle greater than 22.5xc2x0 from the xc3xa8 direction is less than half the maximum gain.
Thus, beam forming forms directivities by phase-rotating signals received from the respective antennas of the array antenna by an arbitrary angle and then combining those signals. That is, by branching the signals received from the respective antenna elements to a plurality of groups and applying different ways of combination thereto, beam forming can form directivities with a high gain in a plurality of different directions. As a result, beam forming can form directivities appropriate for carrying out communications with a plurality of other parties of communication.
For example, it is possible to carry out processing of using beam 24 in the xcex8 direction for communication with mobile station 22 in the xcex8 direction and processing of using beam 25 in the xcfx86 direction for communication with mobile station 23 in the xcfx86 direction simultaneously.
Thus, base station apparatus 21 can receive signals of a plurality of other parties of communication individually by carrying out processing on signals output from the beam former through different reception units provided for the other parties of communication.
However, since the conventional radio reception apparatus estimates the directions of arrival of signals for their respective channels individually, it is not possible to achieve accuracy enough to estimate the directions of arrival due to interference received from other stations under circumstances in which many mobile stations exist adjacent to one another, hence it is not possible to form optimal antenna directivities for reception.
Moreover, to estimate the directions of arrival of radio signals for their respective channels, it is necessary to provide reception units 10-1, 10-2, . . . , 10-N for the respective channels, which requires the same number of direction of arrival estimation sections as channels received by reception units 10-1, 10-2, . . . , 10-N and despreading sections corresponding to (the number of channelsxc3x97the number of antennas), increasing the scale of the circuit and costs.
It is an object of the present invention to provide a direction of arrival estimation method and a radio reception apparatus capable of reducing the scale of the circuit and preventing deterioration of reception quality even if many mobile stations exist adjacent to one another deteriorating the accuracy of direction of arrival estimations of signals.
This object is attained by calculating an intensity distribution of reception signals with respect to the directions of arrival of signals from the reception signals before they are separated according to their respective channels, estimating the directions of arrival of signals in band units (i. e., signals for each band) based on this intensity distribution, determining antenna directivities and receiving signals accordingly, grouping signals coming from quasi-identical directions and estimating the directions of arrival thereof, receiving radio signals with antenna directivities formed in the directions of arrival estimated in group units, and receiving the signals in group units and separating or extracting desired signals.